I don't want to be rude or anything but this looks like it would be a math question
Even though synonyms generally share the same meaning, they might not apply in the same context. So, if you want to use one specific word you should do that instead of trying to find a synonym that doesn't quite fit into the context you've established. An example is if you want to use the sentence "I was mad", meaning "I was angry", you could look for a synonym. One synonym is "absurd". However, this is a different type of mad, meaning crazy instead of meaning angry. "I was absurd" has an entirely different meaning than originally intended. This is why you should always double check the contextual meaning of the synonym you want to use.
One time I investigated that one of my favorite shirt was gone. So, it was a mystery for me i tried to describe it in my head. I remember it in my head and look everywhere at my house and grandma house but it was no where to be found. So, I look downstairs of my grandma house and i sawed in my cousin closet that she had my favorite shirt. So,my mystery was my cousin.
The Awnser is going to be 15
